Most conventional commercial telephony terminals for portable satcom users are those designed for operation in the Inmarsat system. This an L-band service with very large intersatellite spacing (.gtoreq.30.degree.), which easily allows small aperture L-band antennas with 3-Db beamwidths of .+-.6.degree. to be used without excessive interference to the adjacent L-band satellites.
This degree of portability has not been available for other frequency bands, such as C- and K.sub.u -band terminals. This is because for C- and K.sub.u -band applications, the intersatellite spacing is generally 3.degree. and sometimes even 2.degree.. To achieve the very narrow beamwidths required (&lt;&lt;0.5.degree.) for acceptable levels of adjacent satellite interference, VSAT's (very small aperture terminal) need to employ large antenna sizes since the beamwidth is inversely proportional to the dish size. Accordingly, to comply with ITU-R Recommendation 524-3 off-axis radiation emission requirements, VSAT vendors use antenna sizes beginning at 1.8 m for C-band and 1.2 m for K.sub.u -band. For mesh networks where one VSAT directly communicates with another VSAT, the antenna sizes are even larger (e.g., starting at 3.6 m at C-band). Such large sizes increase the terminal cost, severely limit transportability and necessitate elaborate mounting structures for the large dishes.
Portability requires small dish sizes. However, small dish sizes have large beamwidths. For example, a 30-cm K.sub.u -band antenna has a 3-dB beamwidth of .+-.2.degree.. Significant interference to and from adjacent satellites, that are spaced 2.degree. to 3.degree. apart, will occur which will violate ITU-R and U.S. FCC limits on off-axis energy density. For example, at 2.degree. off axis from boresight, ITU 524-3 specifies a K.sub.u -band EIRP density limit of 31.5 dBW/40 kHz. This problem can be solved by using spread spectrum technology, which spreads the transmitted energy over a sufficiently wide band to permit the use of small C- and K.sub.u -band antennas while remaining within the ITU-R off-axis EIRP energy density limits, thus causing no perceptible interference to signals on the adjacent satellites.
For truly portable, rather than merely transportable or luggable, satellite terminals, only diameters of less than 1.5 meters are considered. Traditional single channel per carrier (SCPC) access methods with 0.5 m and less antennas will not allow even one carrier to be supported in the network due to the inbound off-axis interference resulting from the high carrier power concentrated in the narrow band, and the sidelobe gain of the small dish. For a typical INTELSET IV band space segment and a 11 meter diameter hub, SCPC methods require at least a 1.0 m dish in order to support 50 carriers per 18 MHz transponder, while the use of TDM/CDMA allows at least 100 carriers to be supported with 0.5 m dishes. In both cases capacity is constrained due to the satellite transponder downlink power limitations.
The use of spread spectrum and CDMA for small terminals is not new. However, the overall architecture and the reasons for implementing the system structure vary from system to system. One conventional method for performing satellite communications with small dishes deploys star and double-hop mesh small VSATs for packetized data broadcast applications using spread spectrum.
Other methods for telecommunications using spread spectrum and/or CDMA are not relevant to satellite communications because many of these are intended for terrestrial wireless cellular services. For those systems that use spread spectrum for a future LEO satellites, the spread spectrum aspect is used to ameliorate the effects of the fast Doppler frequency variations and frequency selective channel fading on the LEO satellite link. Moreover, conventional small VSAT implementations use dish antennas which are bulky due to their curvature and need a separate feedhorn and feed assembly, as well as an antenna mounting structure.
Conventional star network VSAT systems which employ spread spectrum are intended for one-way data broadcast applications only using a packet-oriented transmissions scheme. Two-way data communications are less frequently used, but still employ a centralized hub earth station as the data center with no specific ability to interconnect the calls to a public switched telephone network (PSTN). In other words, these networks were for business uses, such as credit card approval, inventory monitoring, factory ordering, bank account reconciliation, etc., not for use as a remote extension of the telephone network.
Two-way voice communications in a traditional data packet-oriented VSAT network is cumbersome to implement and manufacturers need to dedicate a separate network mode for voice. Networks offering dedicated voice channels are much more expensive than data-only systems.
Conventional mesh network VSAT systems use CDMA for multiple access to the same frequency band. To allow a large number of users, such systems use wideband CDMA, requiring many MHz of contiguous transponder bandwidth. Either a large PN code set (1000's) or a very long (2.sup.31 -1) PN code is needed, segments of which are used for individual terminals. Since the mesh operation requires direct remote-to-remote operation, the dish sizes are very large, .gtoreq.3.6 m at C-band and .gtoreq.1.8 m at K.sub.u -band.
Network controllers typically operate in a star or mesh mode. A hybrid network controller, that can dynamically configure the frequency plan for a combined star/mesh mode operation, and support both star (from remote to PSTN) and single-hop mesh (remote to remote) terminals has hitherto been unknown. Most VSAT networks have dedicated hubs with vendor-specific RF front-ends.
Accordingly, it is an object of the invention to provide a portable satellite terminal with significantly reduced antenna and terminal size compared to conventional flyaway K.sub.u /C-band satellite terminals, as well as a versatile and flexible hub, accommodating a variety of system architectures.